It has been conventionally possible to display a moving image together with a still image on the same screen. This is called an On-Screen Display (OSD) and is used to display values on a part of the screen when a user sets the brightness of the screen or the like.
In recent years, some digital broadcast reception devices can superimpose a moving image (a program that is being broadcast) with a still image (e.g., a program guide transmitted by a broadcast station), and display them together. Other devices such as video recorders, video players, mobile telephones and digital cameras can also superimpose a moving image with a still image and display them together onto a screen.
One method to superimpose a moving image with a still image is to, after storing the moving image and the still image into different memories, display them by overlaying the still image on top of the moving image.
In this method, the superimposition is performed in such a manner that the still image includes a transparent part that would be positioned over the moving image, so that the moving image can be viewed through this transparent part. Thus, when the display size and position of the moving image are modified, the transparent part of the still image also has to be modified to conform to the new display size and position of the moving image.
Images constituting the moving image are updated at a predetermined frame rate. Accordingly, each time the update is done, the moving image needs to be superimposed and displayed with a still image. Here, if the timing of modifying the display size or the like of the moving image does not coincide with the timing of making the still image partially transparent, then the moving image and the still image would be misaligned when displayed on the screen—in other words, an undesired image would be displayed between borders of the moving image and borders of the still image.
In light of the above problem, there has been developed technology for matching the timing of making the still image partially transparent with the timing of modifying the moving image, so as to prevent the misalignment of the moving image and the still image on the screen (Patent Document 1).
Patent Document 1:    Japanese Laid-Open Patent Application No. 2004-302361
However, with the above technology, it is necessary to store (i) an instruction to make the still image partially transparent and (ii) the timing of displaying a still image whose transparent part has been modified. Furthermore, with the above technology, the processing of matching the stated timings with each other is performed by software; accordingly, depending on the progress made by other applications and the like, there are times when the stated timings do not precisely match each other, due to failure to reserve CPU for a necessary time period.
In view of the above problem, the present invention aims to provide a display device that can, without requiring an extra memory, display a moving image together with a still image without misalignment on a screen.